In general, the roller stands of a seam welded steel pipe manufacturing line is arranged as follows. As illustrated in FIG. 1, the seam welded steel pipe manufacturing line includes: a breakdown section (BD section: 1) in which bending operation conducted on a strip is started; a fin pass forming section (FP section: 2) in which the strip edges are formed into predetermined shapes and the strip is guided into a welding section; a welding section (WA section: 3); and a shape straightening section (SZ section: 4). In the above arrangement, the fin pass forming section 2 is comprised of horizontal and vertical rollers arranged in 3 to 4 stages of roller stands at the end of forming before welding. By the action of a fin plate attached to the vertical roller, the fin roller moves between both edge portions of a strip to be formed and conducts angle-controlling, finish-forming and centering so that both edge portions can be stably formed into predetermined shapes.
As illustrated in FIGS. 2(a) and 2(b), each fin pass forming stand includes: an upper roller 6 having a fin 7 at its center; a lower roller 8 having no fin; and/or side rollers (horizontal rollers) 9. Radiuses of curvature of these rollers and the width of the fin are determined by the outer diameter of the steel pipe 10 to be formed in this manufacturing process. Therefore, according to the conventional fin pass forming method, when the edge portions of a strip are pressed against the fin of the upper roller by the actions of two rollers including an upper and a lower roller, or by the actions of four rollers including an upper, a lower, a right and a left roller, a force for forming is given to the strip in the circumferential direction, so that the edge forming can be conducted while a relatively high reduction is being given to the strip. In the normal operation, the reduction is approximately 1 to 2% in total with respect to all fin pass stands. Usually, a caliber roller is used for the fin pass roller because reduction can be easily given to the strip by the caliber roller.
Therefore, in the case where seam welded steel pipes of a plurality of sizes are formed on one manufacturing line, it is necessary to provide a plurality of sets of rollers, the number of which corresponds to the number of sizes of seam welded steel pipes to be formed. Further, when the size of a seam welded steel pipe to be formed is changed, it takes time to replace the rollers. Furthermore, it is necessary to replace the rollers frequently.
In the field of manufacturing seam welded steel pipes, in accordance with the progress of development of peripheral technology such as technology of manufacturing materials and technology of processing, there are increasing demands for enhancing the accuracy of products, producing a small amount of products of a large number of grades, reducing the number of workers on the production line and automatizing the production line. In order to meet the above demands, it is desired to apply one roller device to seam welded steel pipes of a plurality of sizes. Also, it is desired to ensure a stable welding condition in the welding process by improving the forming method of seam welded steel pipes.
Concerning the fin pass forming, various techniques have been developed so as to satisfy the above demands, however, the development is limited to improvements in the shape of the fin. An example of improvements in the shape of the fin is disclosed in Japanese Unexamined Patent Publication No. 5-277567, which will be described as follows. Instead of a shape of the conventional upper roller having a fin, a special shape of the roller, which is formed into a fin-plate shape, is adopted. There are arranged upper rollers so that they can be respectively tilted and rotated on both sides of this fin plate. In accordance with the state of the upper rollers which are tilted and rotated, only linear or curved caliber surfaces of the fin plate are contacted with both edge portions of the strip to be welded. Therefore, the shapes of both edge portions of the strip can be easily changed in the process of forming, and the welding condition can be changed in accordance with the shape of both edge portions of the strip.
Japanese Unexamined Patent Publication No. 3-169432 discloses the following forming method. In order to bend the edge portions of a strip sufficiently so as to form the edge portions into predetermined shapes and increase the thickness of the edge portions while utilizing the strong drawing effect in the front stage of the fin pass rollers, fin pass forming is conducted by the front two fin pass roller stands, and the thus formed shape is maintained by the residual roller stands in the rear stage so as to weld and convey the edge portions. Due to the foregoing, all the residual roller stands in the rear stage are made to be available for forming seam welded steel pipes of a plurality of sizes.
However, all the above techniques have the following problems. According to the conventional fin pass rolling method, the number of roller stands is 3 to 4, and in each roller stand, the rollers are arranged at the upper and the lower position, or alternatively they are arranged at the upper, the lower, the right and the left position. On the upstream side of the fin pass rollers, there are provided side rollers so as to conduct an auxiliary fin pass rolling. Therefore, rollers in each roller stand must be set for each size of the steel pipe to be formed. Further, in the case of replacing the rollers, it is necessary to adopt a cassette system roller replacing method by which the rollers in each roller stand are replaced or the rollers in a plurality of roller stands are replaced simultaneously.
Further, the following problems may be encountered. According to the conventional steel pipe forming method, edge bending can not be controlled sufficiently in the breakdown pass of the front stage. Accordingly, it is necessary to give a large amount of reduction in the fin pass rolling process so as to make up for edge bending. Accordingly, it is impossible to avoid the deformation of a steel pipe to be formed in the process of fin pass forming, so that the roundness of the steel pipe to be formed can not be ensured.
From the viewpoint of recent tendency of the production system in which a small quantity of products of a large number of grades are produced and production is carried out without having stock, the roller replacing operation described above takes time and labor, so that the productivity is greatly deteriorated.
Conventionally, seam welded steel pipes are produced as follows. A strip to be formed is subjected to edge forming in the breakdown process. Then the strip is subjected to elliptical forming by the cluster mill. After that, in the fin pass rolling process, angles of the edge surfaces are controlled by the fin rollers, and finish rolling and centering are conducted. After an amount of upset necessary for welding is adjusted by the squeeze rollers, welding is conducted on the edge portions. In the sizing process, the thus formed steel pipe is straightened, and the pipe is finished to a final product. In the above producing process, the cluster process and the fin pass rolling process are usually conducted by different roller stands. The reason is described as follows. In the process of deformation in the thickness direction of a strip to be formed, on the external surface of the strip, tension/compression is given, and on the internal surface, compression is given. Accordingly, different roller stands must be provided in accordance with the states of deformation conducted on the strip. Therefore, rollers must be replaced for each product size, and further a plurality of roller stands must be prepared for replacing the roller stands. Furthermore, it takes time and labor to replace the roller stands. In addition to that, the equipment investment is doubled.
Calibers of the conventional cluster rollers available for forming seam welded steel pipes of a plurality of sizes are designed so that they can be applied to the seam welded steel pipes of large and small sizes. Therefore, the section of the cluster roller is formed from an involute based on a polygon capable of being applied to various pipe sizes, and when the cluster roller is turned, it can be applied to seam welded steel pipes of a plurality of sizes. Due to the foregoing, when the pipe size is changed, it is natural that a contact position of the strip with the roller caliber is changed, especially it is natural that a contact position of the edge with the roller is changed. For this reason, it has been considered that it is impossible to provide a shoulder stop of the edge portion of the strip to be formed, that is, it has been considered that it is impossible to provide a fin roller. Accordingly, from the recognition that the cluster rolling process and the fin pass rolling process must be provided being separate from each other, they are arranged as different apparatus. Therefore, the aforementioned problems have not been solved yet.